The present invention relates to circuits for the transmission of telephone signals, in particular monolithically integrated transmission circuits designed to be inserted in the speech circuits of subscribers' telephone sets.
A subscriber's telephone set is, in general, connected to the telephone exchange by a two-wire line to whose terminals the speech circuit, which is designed to receive and transmit the telephone signals via the line, is connected in parallel with other telephone components.
The speech circuit is supplied by the line in which there flows a direct current on which an alternating signal component is superimposed.
The various stages of the speech circuit are designed such that the current absorption from the line for their power supply is constant so as not to interfere with the signals themselves.
A telephone speech circuit is essentially a signal transducer which enables the connection of both the microphone transmitter and the receiver of the subscriber's telephone set to the subscriber's telephone line.
In general, the speech circuit of a subscriber's telephone set is constructed using a structure having a Wheatstone bridge configuration.
One "diagonal" of the bridge forms the reception channel of the circuit and the other the transmission channel.
The terminals of the subscribers' telephone line are connected to two nodes of the bridge which do not belong to the same diagonal.
Accurate balancing of the entire bridge structure is essential in order to minimize the "local effect", i.e. the effect in which the person speaking hears his or her own voice through the receiver of the telephone set.
However, in speech circuits which are partly or totally integrated, completely accurate balancing may not be sufficient to provide control over the local effect for correct operation, the stages of a monolithically integrated telephone circuit require a minimum supply voltage whose value is determined by the design layout and the particular integration technology used.
The maximum voltage which may be used for the supply of the stages of a telephone speech circuit is the voltage at the terminals of the line to which the circuit is connected which is equal to the voltage of constant value of the line present at these terminals in the absence of a signal plus a variable voltage component due to the possible signal being received and transmitted and superimposed on this voltage of constant value.
Since the signals being received are attenuated by the line when they reach its terminals, it is essentially the signals being transmitted, amplified by the speech circuit, which cause positive or negative variations of the voltage present at the line terminals.
When the direct voltage at the terminals of the line is reduced as a result of the length of the line, the instantaneous voltage at the line terminals may be, for one or more stages of the speech circuit, lower than the minimum voltage required for the correct operation of these stages, in the area of the negative peaks of the waveform of the signal being transmitted.
This has a detrimental effect on the correct operation of the entire speech circuit, particularly if it is monolithically integrated, with considerable interference in both transmission and reception.
The overall balance of the circuit is obviously lost with the result that control over the "local effect" is compromised.
The possible occurrence of these drawbacks in subscribers' line of considerable length has, to a large extent up to now, prevented the general introduction of monolithically integrated speech circuits in subscriber's telephone sets.